Keep track of parse errors in mods and don't emit resolve errors for paths involving them

When we expand a `mod foo;` and parse `foo.rs`, we now track whether that file had an unrecovered parse error that reached the end of the file. If so, we keep that information around. When resolving a path like `foo::bar`, we do not emit any errors for "`bar` not found in `foo`", as we know that the parse error might have caused `bar` to not be parsed and accounted for.

When this happens in an existing project, every path referencing `foo` would be an irrelevant compile error. Instead, we now skip emitting anything until `foo.rs` is fixed. Tellingly enough, we didn't have any test for errors caused by `mod` expansion.

Fix #97734.

compiler/rustc_ast/src/ast.rs

@@ -2877,7 +2877,7 @@ pub enum ModKind {
     /// or with definition outlined to a separate file `mod foo;` and already loaded from it.
     /// The inner span is from the first token past `{` to the last token until `}`,
     /// or from the first to the last token in the loaded file.
-    Loaded(ThinVec<P<Item>>, Inline, ModSpans),
+    Loaded(ThinVec<P<Item>>, Inline, ModSpans, Result<(), ErrorGuaranteed>),
     /// Module with definition outlined to a separate file `mod foo;` but not yet loaded from it.
     Unloaded,
 }

compiler/rustc_ast/src/mut_visit.rs

@@ -1212,7 +1212,12 @@ impl WalkItemKind for ItemKind {
             ItemKind::Mod(safety, mod_kind) => {
                 visit_safety(vis, safety);
                 match mod_kind {
-                    ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
+                    ModKind::Loaded(
+                        items,
+                        _inline,
+                        ModSpans { inner_span, inject_use_span },
+                        _,
+                    ) => {
                         items.flat_map_in_place(|item| vis.flat_map_item(item));
                         vis.visit_span(inner_span);
                         vis.visit_span(inject_use_span);

compiler/rustc_ast/src/visit.rs

@@ -380,7 +380,7 @@ impl WalkItemKind for ItemKind {
                 try_visit!(visitor.visit_fn(kind, span, id));
             }
             ItemKind::Mod(_unsafety, mod_kind) => match mod_kind {
-                ModKind::Loaded(items, _inline, _inner_span) => {
+                ModKind::Loaded(items, _inline, _inner_span, _) => {
                     walk_list!(visitor, visit_item, items);
                 }
                 ModKind::Unloaded => {}